There are some folks who can throw together a couple dozen routers, a few firewalls, and a load balancer or two; and connect them together with a combination of MPLS, T1s, OSPF, BGP, and bubblegum.
And then there are the rest of us.
GNS3 is a really powerful tool for testing out network ideas, designs, and concepts. It also has uses for monitoring engineers, when we want to try out an alert or a new SNMP OID, but simply don't have the gear. But the problem is that to set up a network in GNS3, you actually have to KNOW how to network. And a bunch of us don't.
So I've uploaded a very simple GNS3 network that you can use to get started. It's 3 routers (Cisco 3600's) configured to connect using EIGRP.